A helicopter generally has a single main rotor driven mechanically by at least one turbine engine, the main rotor serving to provide the helicopter with lift and propulsion.
That type of helicopter is then provided with a tail rotor that performs an anti-torque function by exerting transverse thrust in order to compensate the torque created by rotation of the main rotor.
In addition, the tail rotor enables the pilot to control yaw and steering maneuvers of the helicopter by exerting transverse thrust that is positive or negative.
A non-shrouded tail rotor, referred to herein as a “conventional” tail rotor for convenience, is known that performs this function. Conventionally, a non-shrouded tail rotor is mounted on one side of the tail fin and at its top end, or on an end of the tail boom of the helicopter.
Such non-shrouded tail rotors are in widespread use. Nevertheless, it is also possible to implement a shrouded tail rotor, known under the trademark Fenestron®.
The shrouded tail rotor is then arranged in a duct formed through the bottom portion of the vertical fin of the helicopter tail, the longitudinal axis of the duct being substantially perpendicular to the vertical plane of symmetry of the helicopter.
Consequently, the streamlined structure of the vertical tail fin of the helicopter surrounds said duct and thus the tail rotor, which explains why it is referred to as a shrouded tail rotor. It should be observed that this streamlined structure is generally called a “shroud” by the person skilled in the art.
The streamlined structure then protects the shrouded tail rotor against impacts. Similarly, it increases safety for personnel on the ground by preventing such personnel being injured by the shrouded tail rotor.
In addition, from an acoustic point of view, a shrouded tail rotor constitutes significant progress compared with a tail rotor that is not shrouded.
The streamlined structure surrounding the duct containing the shrouded tail rotor prevents noise from escaping towards the front and towards the rear of the rotorcraft, whereas a non-shrouded tail rotor broadcasts noise in all directions.
Consequently, using a shrouded tail rotor reduces the sound nuisance of a helicopter.
Document FR 2 719 550 describes a shrouded tail rotor that is optimized from an acoustic point of view.
The shrouded tail rotor is arranged in an air flow duct having an axis extending transversely relative to the helicopter, with a flow-straightening stator being secured in the air flow duct upstream from the shrouded tail rotor.
That shrouded tail rotor has a plurality of blades that are distributed irregularly such that the angle between any two blades of the shrouded tail rotor is different from the angle between any two vanes of the stator.
Nevertheless, it is found in practice that a shrouded tail rotor continues to give rise to a non-negligible amount of noise, in particular when the helicopter is traveling at medium speed, above 80 knots, for example.